Short pulse, high power lasers are used in various applications, since these lasers have power advantage of a pulsed laser over a continuous output laser is that the energy output can be compressed into a very short time period, resulting in very high energy per unit time. Two traditional ways of achieving short optical pulses with high optical power are cavity Q-switching and cavity mode locking. In the Q-Switch method the Q-factor or figure of merit for a cavity is initially set very low such that energy transferred into the cavity does not induce appreciable stimulated emission. The energy within the cavity is allowed to build up until the Q-factor of the cavity is switched rapidly to a very high state such that significant feedback is present, significant stimulated emission is generated, and the cavity lases, thereby generating an intense, short optical pulse that discharges a substantial portion of the energy the cavity had stored during the low-Q state. The Q-switch technique allows substantially more energy to be stored and released by the resonant cavity than if the resonant properties of the laser cavity were not reduced by the Q-switch. The Q-switch may be accomplished by the use of non-linear crystals, saturable absorbers, and oscillating or rotating mirrors. Q-switched lasers are described, for example, in LASE 2004 Conference Proceedings 5332, and in J. Nettleton, et. al. “Monoblock laser for a low-cost, eyesafe, microlaser range finder”; Applied Optics, Vol. 39, No. 15, 20 May 2000, pp. 2428-2432. Cavity mode locking also utilizes a laser, but instead of a Q-switch, the longitudinal modes of the laser are locked to a set spectral spacing such that the superposition of the broad spectrum of individual spectral peaks superimposes to create an optical signal that is narrow in the time domain. Cavity mode locking is described, for example, in Siegman, LASERS, University Science Books, January 1986, ISBN 0935702113.
Although providing very short optical pulses, lasers operating by Q-switching and cavity mode locking are expensive, large and bulky, and are often custom built for optical applications. Thus, other ways have been developed for achieving short high power optical pulses using a cladding pumped fiber. Cladding pumped fiber lasers use a specially prepared glass fiber having a core on the order of 5 micrometers diameter doped with rare earth ions, such as Erbium (Er), Neodymium (Nd), or Ytterbium (Yb). The surrounding glass cladding which supports the doped core is irradiated longitudinally (along the fiber axis) by high power pumping lasers whose wavelength is selected to be absorbed by the rare earth dopants, and whose combined power may be many kilowatts. The cladding is much larger in cross section than the core, so that much more optical power can be injected than could be injected directly into the core. As the high power pumping laser beams cross the core of the glass fiber they are not captured to form a guided wave, but nonetheless are partially absorbed to energize the rare earth ions. The cladding is intentionally fabricated so that it is not round, and the pumped laser light undergoes mode-mixing in the glass fiber to avoid depleting the modes that intersect the core. Using this method, IPG Photonics of Oxford, Mass., USA, produces lasers that exceed 10 kilowatts continuous output in a beam diameter of 100 micrometers. See also for example, U.S. Pat. No. 5,949,941, issued to D. J. DiGiovanni, titled “Cladding-pumped fiber structures”.
A different class of actively pumped fiber often used in optical communication systems injects the pumping laser beam directly as a single mode beam into the doped core of the glass fiber. This often limits the amount of pump power that can be injected to many hundreds of milliwatts. Such a device is often referred to as an optical fiber amplifier, since it is typically used to amplify optical signals of the proper wavelength as they pass through the core of the fiber. These amplifiers are generally operated in the linear regime, where the input signal is small enough that the gain is independent of the signal. In this linear region, the output signal is an amplified exact replica of the input signal in terms of wavelength, polarization state, power vs. time. For example, such optical amplifiers are described in Waarts et al., U.S. Pat. Nos. 6,081,369, 5,933,271, and 5,867,305. The goal of such optical amplifiers is to produce an output optical signal that is an amplified version of an input optical signal, and as such it is important to maintain fidelity of the temporal shape of the output signal with respect to the input signal. For the case of a series of input optical pulses, these optical amplifiers provide output optical pulses having the same wavelength, pulse width, and spacing between pulses as the input optical pulses, but at an increased optical power level. Although useful in optical communication systems to linearly amplify communication signals, the release of stored energy from the optical fiber must be over the entire input signal to linearly amplify such signals, which limits applications of such amplifiers.
Although optically pumped fiber are useful to transfer energy coherently into optical signals, i.e. to optically amplify the optical signals, they have not heretofore been used with trigger pulses as described by the present invention which causes a cascade energy release of stored energy in the form of high power optical pulses that more efficiently utilize the stored energy, rather than to amplification of the entire optical pulse that triggered such release. Thus, it would be desirable to produce short high power optical pulses without the complexity and expense of Q-switching and cavity mode locking based devices, which are initiated by such optical trigger pulses applied to a pumped energy storage medium, where each trigger pulse produces a cascade energy release as an output optical pulse that is not an amplified replica of the trigger pulse or any other input optical signal.